The present invention relates to a coordinate data entry device for entering data of a position indicator on a board surface by the use of a surface entry type acoustic coordinate data entry technology utilizing ultrasonic waves. (The foregoing xe2x80x9ccoordinate data entry technologyxe2x80x9d is referred to as a xe2x80x9cdigitizing technologyxe2x80x9d, wherever appropriate, hereafter.)
In recent years, a greater use of a digitizer has been prevailing in picking up coordinate data by pointing a position indicator at a design drawing posted on a drawing board, thereby allowing the coordinate data of the design drawing to be processed for input to a personal computer (referred to as PC, hereafter). A position indicator is provided with the function of transmitting ultrasonic waves and the transmitted ultrasonic waves are received by the receivers installed on the upper edge of the drawing board at both ends thereof, thereby allowing the point positions of the position indicator on the drawing board to be transmitted to PC for transforming the positional information to data.
FIG. 4 is a schematic illustration of a prior art coordinate data entry device, FIG. 5 is a schematic circuit diagram of an ultrasonic oscillating circuit used in a prior art coordinate data entry device, FIG. 6 shows signal waveforms of a prior art coordinate data entry device and FIG.7 illustrates how the erroneous performance of a prior art coordinate data entry device is caused by reflective waves.
The coordinate data entry device of FIG. 4 comprises position indicator 7 to transmit ultrasonic waves intermittently and receivers 6 mounted on the upper edge of drawing board 5 at the right and left ends thereof. Receiver 6 measures a propagation period of time required of the ultrasonic waves from position indicator 7 to reach receiver 6 and converts the measured propagation period of time to a corresponding distance, finds out the coordinate of position indicator 7 according to a triangulation technique and transmits the positional information to PC 8. For an accurate time measurement, position indicator 7 and receiver 6 are synchronously operated all the time and, upon transmitting ultrasonic waves from position indicator 7, receiver 6 starts a time measurement in synchronization with position indicator 7, thereby enabling the measurement of propagation time to be carried out accurately.
With the coordinate data entry device having the foregoing structure, the positional information of position indicator 7 is displayed instantaneously on the display screen of PC 8 and the plotting information on drawing board 5 is allowed to be entered in PC 8.
Next, a reference is made to a prior art ultrasonic oscillating circuit that is built in position indicator 7 of the aforementioned coordinate data entry device. FIG. 5 and FIG. 6 show a schematic circuit diagram and signal waveforms of the ultrasonic oscillating circuit, respectively. As FIG. 5 shows, the ultrasonic oscillating circuit comprises transistor 1, ultrasonic resonator 4 and pulse transformer 9. When a pulse of FIG. 6A is fed to transistor 1, an electric current flows in the primary winding of pulse transformer 9, thereby causing a stepped up voltage to be generated in the secondary winding. When the pulse application ceases thereafter, a damped oscillation with the stepped up voltage serving as the reference voltage starts in the resonant circuit formed of the secondary winding of pulse transformer 9 and ultrasonic resonator 4 and an output voltage having the waveforms as shown in FIG. 6B is applied to ultrasonic resonator 4, resulting in generation of ultrasonic waves.
However, with the coordinate data entry device of FIG. 5, even if the pulse fed to transistor 1 remains the same in magnitude, variations in the level of generated ultrasonic waves are unavoidable because of the variations involved with the performance of the power supply and pulse transformer and also the output of the ultrasonic resonator.
As FIG. 5 shows, a damped oscillation voltage is applied to the ultrasonic resonator contained in a position indicator and the ultrasonic waves actually propagated in the air exhibit the waveforms with the amplitude thereof increased and then attenuated gradually. (See FIG. 6C) In addition, since a resonance type ultrasonic sensor is in general used in receiver 6, the received waveforms show a gradual increase in amplitude and then a trailing amplitude attenuation after passing the maximum amplitude as FIG. 6D shows. Generally, in order to minimize the amount of deviation in positional detection, receiver 6 for receiving ultrasonic waves is arranged to detect the level of a waveform after a few waveforms are passed since the start of receiving the ultrasonic waves by means of a comparator, the threshold level of which is set up at the aforementioned level, thereby allowing the period of time between the start of ultrasonic waves transmission and the arrival of ultrasonic waves at receiver 6 to be measured. In this case, a variation of oscillating ultrasonic waves in level ends up with causing the detection position to shift erroneously. When the detection position shifts by one wave (one wavelength), the frequency of the ultrasonic waves is 40 KHz and the velocity of sound is 340 m/s, the error involved with the distance measurement value turns out as large as 8.5 mm.
Further, as FIG. 6D shows, the ultrasonic waves reach the maximum amplitude at a position behind the detection position due to a resonance phenomenon and, when the driving of the ultrasonic resonator is performed intermittently with these waveforms, the reflective waves received with delay cause a problem. A description is given to this problem with reference to FIG. 7. The upper diagram of FIG. 7 shows the ultrasonic waveforms of an intermittent oscillation while the lower diagram of FIG. 7 shows the waveforms of received ultrasonic waves. When the ultrasonic waves are received, receiver 6 receives both the direct waves that arrive at receiver 6 via the shortest possible distance and the reflective waves that are attenuated and delayed due to bouncing off an object.
As FIG. 7 shows, when the reflective waves are received after the next oscillation waves and the maximum amplitude thereof exceeds the threshold level of the comparator, the propagation period of time of the ultrasonic waves is txe2x80x2 instead of t, thereby making the propagation period of time shorter and allowing the step of positional detection to be performed erroneously.
The present invention provides a coordinate data entry device comprising:
a plurality of receivers disposed in the vicinity of a board surface; and
a position indicator to transmit signals, at least one of which employs ultrasonic waves, to the receivers, thereby detecting the position thereof on the board surface and making the coordinate position thereof available for entry,
in which the signal employing ultrasonic waves is transmitted at an arbitrary time interval from an ultrasonic oscillating circuit comprising a resonant circuit formed of an ultrasonic resonator and a coil, which are connected in parallel with each other, and
the magnitude of an electric current, which starts the operation of the resonant circuit, to vary according to the lapse of time.